This invention relates to systems for providing a constant and accurate signal gain with relatively wide bandwidth based on an signal amplifier with high frequency response having it's gain controlled by a gain control amplifier.
In general, the prior art systems have used negative feedback between the output and input of the signal amplifying circuit for obtaining an accurate constant signal gain. A major problem in such systems has been that to maintain signal amplifier stability the signal amplifier frequency bandwidth needed to be limited to obtain relatively large gain and phase margins.
The present invention overcomes the bandwidth limitation of the negative feedback amplifying circuit by allowing the input signal to be amplified by a separate amplifier (signal amplifier) than the amplifier (gain amplifier) that controls the gain applied to the signal. The gain amplifier can have relatively low frequency response compared to the signal amplifier since amplifier gain changes are normally slow in comparison. The gain amplifier compares the input signal level to a signal proportional to the signal amplifier output signal level to obtain a gain control signal. This gain control signal is applied as negative feedback to control the gain applied to the input signal. Thus the present invention does not have the same limited frequency bandwidth requirements to maintain large gain and phase margins.
An objective of the present invention is to provide a constant and accurate signal gain system that has a high temperature, radiation, and voltage stability due to its reliance on passive component ratios to set circuit gain values. Passive components such as resistors and capacitors are more stable under these conditions. This invention increases system accuracy by making the accuracy dependent on passive component ratios instead of transistor stability.
Another object of the invention is to provide bandwidth improvement in constant gain signal amplifiers.
A further objective of the invention to provide circuits that are less susceptible to process variances by relying on impedance ratios thereby providing a more consistently manufacturable circuit.